Possessed of excellent mechanical properties, fatigue resistance, resistance to friction and wear, resistance to chemicals, and moldability, polyacetal resin has found application in a variety of fields such as automotive parts, electrical and electronic parts, other precision machine parts, architectural members and pipeline components, household and cosmetic parts, and medical device parts. However, with the increasing scope and diversification of uses, the resin is required to satisfy more and more sophisticated requirements. The necessary performance profile of polyacetal resin is that it does not experience reductions in mechanical strength during processing such as extrusion or other molding, does not stick to the die or mold (no mold deposits), retains its mechanical characteristics fully even upon prolonged heating (no heat aging), does not give rise to molding defects such as silver streaks and voids, and does not liberate formaldehyde after molding. One of the factors causing those phenomena is the degradation of the polymer on heating. Particularly because of its chemical structure, polyacetal resin is inherently unstable in an oxidative atmosphere at an elevated temperature or in an acidic or alkaline environment. Therefore, the essential need that must be fulfilled for polyacetal resin is that of insuring high thermal stability and minimal emission (or generation) of formaldehyde in the course of processing and from shaped articles. Formaldehyde is chemically active and ready to be oxidized to formic acid to thereby adversely affect the heat resistance of resin and, when the resin is used as electrical or electronic parts, cause corrosion of metallic contacts or their discoloration due to organic deposits, resulting in contact errors. Furthermore, formaldehyde as such contaminates the working environment for parts assembling and the ecology in the field of use of end products.
As means for stabilizing the chemically active end groups of the homopolymer, there is known a technique which comprises esterifying the polymer end groups by, for example, acetylation and, in the case of a copolymer, there is known a technology which comprises copolymerizing trioxane with a monomer having a neighboring carbon bond such as a cyclic ether or a cyclic formal in the stage of polymerization and then cleaving off the unstable end groups to generate inactive and stable end groups. However, the polymer suffers also from cleavage and decomposition of the main chain on heating and either of the above technologies alone is not effective enough to prevent this decomposition. Actually, therefore, it is considered essential to add an antioxidant and other stabilizers.
As a means for inhibiting emission of formaldehyde from polyacetal resin, it is known to add an antioxidant such as a hindered phenol or a hindered amine, as well as other stabilizers such as nitrogen-containing compounds, e.g. urea derivatives, guanidine derivatives, melamine derivatives, amidine derivatives, polyamides, polyacrylamide, etc., alkali metal hydroxides, alkaline earth metal hydroxides, organic or inorganic salts, and so on. Among the above-mentioned stabilizers, melamine derivatives are comparatively more effective. Moreover, antioxidants are generally used in combination with other stabilizers.
However, even with such additives, it is difficult to completely prevent decomposition of polyacetal resin or impart sufficiently high thermal stability to the resin to suppress emission of formaldehyde in any significant measure. Actually, the resin is subjected to the action of heat and oxygen within the extruder or other molding machine cylinder barrel in the course of melt-processing such as extrusion for the preparation of a composition or production of moldings. Therefore, formaldehyde is inevitably generated due to decomposition of the main chain or from the end groups which have not been sufficiently stabilized, so that the working environment for extrusion molding is adversely affected. Furthermore, on prolonged molding, fine particles and tarry matter adhere to the metal mold (mold deposits) to detract from working efficiency and affect the surface condition of moldings. Moreover, because of their tendency toward an increased risk for mold deposits and blooming (or bleeding), those additives cannot be incorporated in sufficiently large amounts. Furthermore, decomposition of the polymer results in reduced mechanical strength and causes discoloration. The above-mentioned technology is not sufficiently effective in inhibiting emission of formaldehyde from a polyacetal resin composition not only in the course of its processing but also from articles molded therefrom. Therefore, in the fields of motor vehicles, electrical and electronic devices, architectural members and pipeline components, household and cosmetic parts, and medical devices which are closely associated with human living and activity, the industry is required to further reduce the emission of formaldehyde from shaped articles which are end products. In view of the foregoing, a great deal of effort is being expended to find a more effective stabilizing formula for polyacetal resin.
Japanese Patent Publication No. 50502/1980 (JP-B-55-50502) and Japanese Patent Application Laid-open No. 73267/1994 (JP-A-6-73267) propose the use of super-macromolecular melamine derivatives available on melamine-formaldehyde polycondensation for enhanced heat stability and improvements in the risk for mold deposits and blooming. However, even with such a super-macromolecular melamine derivative, it is still difficult to achieve any remarkable inhibition of formaldehyde emission.
Japanese Patent Application Laid-open No. 88136/1973 (JP-A-48-88136) discloses a polyacetal composition containing a stabilizer comprised of a phenol compound and a nitrogen-containing compound such as hydantoin or its derivative for improved thermal stability and resistance to oxidation of polyacetal. However, even by adding such a hydantoin compound, it is still difficult to control the emission of formaldehyde from polyacetal resin at a sufficiently low level.
The present invention has for its object to provide a polyacetal resin composition adapted to insure improvements in the heat stability of polyacetal resin, particularly the melt-stability of the resin in molding, and to a process for its production.
It is another object of the invention to provide a polyacetal resin composition conducive to a marked inhibition of formaldehyde emission at a low level of addition and contributory to improvements in the living and working environments, a process for its production, and shaped articles as molded therefrom.
It is a further object of the invention to provide a polyacetal resin composition adapted to inhibit emission of formaldehyde even under severe conditions to suppress deposition of decomposition products on the mold and blooming or bleeding of such products from a shaped article and thermal aging or deterioration of the article, thus contributing to upgrading of the moldability and quality of the shaped articles, and a process for its production.
It is a still further object of the invention to provide shaped articles of polyacetal resin which have been remarkably inhibited against emission of formaldehyde.
A further object of the invention is to provide polyacetal resin moldings or articles suited for use in the fields of motor vehicles, electrical and electronic parts, architectural members and pipeline components, household and cosmetic products, and medical devices where the emission of formaldehyde is subject to rigorous regulatory control.